Over the past few years, instruments have been developed which enable a target point seen from a CAT (Computerized Axial Tomography) scan slice to be related precisely in space to an instrument or guidance system attached to the body of the patient that is being scanned. Such a system is referred to as a localizer system. Until this present invention, localizer systems have enabled image points to be determined only for axial slices, which are the type that the CAT scan provides. Now, however, the NMR (Nuclear Magnetic Resonance) and PET (Position Emission Tomography) scanners enable scan slices in nearly any arbitrary plane. The present invention is related to determining target coordinates from such arbitrary plane slices.
Preliminary to describing the invention, the definitions of axes and planes will be given. FIG. 1 shows a head and an x,y,z coordinate system superposed. The body axis is the Z-axis, and a planar slice perpendicular to that axis will be referred to as an axial slice or plane. Of course, there is no precise body axis, so these terms refer to axes and planes that are substantially parallel and perpendicular to the body axis, respectively. They may be precisely defined relative to a frame fixed to the body, the axial axis of said frame being approximately parallel to the body axis. The x and y coordinate axes are defined to be perpendicular to the sagital and coronal planes, respectively, as shown in the figure. In the past for CAT scan images, axial plane slices were standard. Now with new generation CAT, NMR, PET and other scanners, slices in the sagital, coronal, or other arbitrary planes can be taken. Note that the sagital plane is one parallel to a plane roughly going through the mid-line of the head (i.e. nose and midway between the ears). The coronal plane is one parallel to a plane that goes through the ears and up over the crown of the head.
The way in which localizers have determined exact coordinates in axial scans to date is illustrated in FIG. 2. Here a head ring 1 is fixed to the skull by screws 2. This provides a rigid apparatus fixed relative to the body. Now, fastened to 1 is a rod system, with rods 3,4,5,6 being essentially parallel to the body axis 2. Between them are diagonals 7,8,9,10. When an essentially axial scan cut 11 is taken through the head, then the rods and diagonals will appear as localizer image spots on the CAT scan image, points 3',4',5',6' and 7',8',9',10' in FIG. 3a. In addition, one might identify a target spot 12 on the CAT image of FIG. 3a. By knowing the proportional distances of the diagonal points from their respective adjacent rods in the image of FIG. 3a, and knowing their actual physical positions on the localizer frame of FIG. 2a, it is possible to calculate the x,y,z positions of each of the diagonal intersection points 7",8",9", and 10" of the diagonal rods and the scan plane as shown in FIG. 2a. These enable determination of the equation of the plane 11 relative to the x,y,z coordinate system which is related to the frame 1. Also by proportional vector calculations, one can then determine, from the position of the target image 12 in the CAT image, the x,y,z coordinates of the target relative to the frame 1. Note that one needs at least the (x,y,z) of three of the diagonal intersections 7",8",9", and 10" to determine the plane 11, so that only 3 diagonals are really required--the fourth may be used as a check or for greater accuracy. Note also that if the frame 1 is clamped parallel to the scan plane 11, then one needs only one diagonal of the four shown to determine the z position of the plane and, thus, the equation of the plane. The version in FIG. 2a is the scheme of the Leksell stereotaxic guide localizer system, in that case using only 2 diagonals and clamping the frame 1 parallel to the scan plane 11.
It is noted perpherally that once the x,y,z coordinates of the target are known relative to the frame 1, then a stereotaxic guidance system can be attached to the frame and an instrument directed to precisely reach the target. This is one of the principal uses of the localizer system i.e., as part of a stereotaxic guide.
FIG. 2b shows another commercially available system, the BRW Brown-Roberts-Wells localizer system. It utilizes six axial rods and three diagonals, thereby determining the three diagonal-plane intersection coordinates, and thus the scan plane, even if the scan plane is not parallel to the frame plane.
FIG. 3b shows a corresponding scan plane image with rod, diagonal, and target image points.
In none of these systems would it be possible to determine x,y,z coordinates of a target if the scan plane were substantially in the sagital or coronal orientations such that the axial rods and their respective diagonals were not cut by the scan plane.
Thus it is an object of this invention to provide a localizer system which can determine target coordinates not only for essentially axial scan cuts, but also for cuts which are substantially parallel to the sagital or coronal planes, or, moreover, for any planar cut through the subject.